1. Field of the Invention
The present invention relates to a method and a device for measuring vibrational velocities and stresses of waves propagated within formations surrounding a wellbore and more particularly elastic waves propagated within formations to be explored.
2. Description of the Prior Art
Determination and knowledge of certain physical characteristics of the subsurface geological strata make it possible to obtain valuable information for identifying a sedimentary rock formation during seismic exploration, for example.
One of the methods of exploration consists in transmitting elastic waves within the medium to be explored, in receiving in the form of signals the waves reflected and/or refracted by the different reflectors of the medium, then in processing the signals in order to extract information relating to the geological strata in which said waves have been propagated.
A method of this type consists in making use of a tool which is lowered into a wellbore. The tool can be provided solely with receivers whilst the transmitters are located at the surface of the medium to be explored. Alternatively, the tools can be both transmitters and receivers.
The propagation of an elastic wave within the formations surrounding the wellbore produces local strain within these formations, with the result that the particles constituting the medium in a measuring zone or point move away from their position of equilibrium under the action of a stress which is of the vibrational type and at a velocity which is also vibrational.
The velocity and strain waves thus generated are in phase opposition and differ in amplitude. The ratio of amplitude of the strain wave to the amplitude of the velocity wave, changed in sign, is equivalent to a so-called elastic impedance.
Now the determination of elastic impedances of a medium in which a wave is propagated would permit a considerable improvement in what is now customarily designated as formation evaluation in the medium traversed by a wellbore and would make it possible to obtain a better knowledge of certain physical parameters such as, for example, the density of the rock formation and/or the different moduli of elasticity of a given formation and consequently of all the rocks of formations through which the wellbore extends.
Up to the present time, little attention has been given by specialists to direct determination of elastic impedances of a medium for the essential reason that it has proved difficult to make use of conventional means for its practical realization and that, when it was in fact found possible to do so, this direct method was vitiated by too great a degree of error to permit of its being considered as a reliable parameter.
In fact, the practice adopted in terrestrial seismology consists in measuring the vibrational velocity since the measurement is made at the ground surface. Under these conditions and by reason of the fact that the ground is a free surface, the stress is zero (stress node). In ocean seismology, an accessible measurement is provided by vibrational pressure since it is not possible to take up a position at the surface of the water, the measuring units being submerged to a depth of a few meters below the free surface of the water. Consequently, and because surface seismology is involved, it is not possible to gain access to the measurement of elastic impedance of formations in which the transducers are located. In surface reflection seismology, the measurement of reflection coefficients permits access to the impedance contrasts of two adjacent strata but never directly to the impedance of each of these strata.
Moreover, it is necessary to recall that specific transducers for measurement of pressure alone and of velocity alone have different pulse responses. As a result, the values delivered by these different transducers cannot be employed for determination of an elastic impedance which would be excessively erroneous.